[LoongArch] Specify registers used for exception handling

See definition in backend D134709 and the doc [1] for more detail.

With the benefit of this change, most libcxx and libcxxabi tests pass.

[1]: https://llvm.org/docs/ExceptionHandling.html

Reviewed By: xen0n, wangleiat

Differential Revision: https://reviews.llvm.org/D139177